This invention relates to dielectric block filters for radio-frequency signals.
Monoblock conductively-coated dielectric filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic monoblock filter design, the resonators are formed by typically cylindrical passages, called through-holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the through-holes.
One of the two opposing sides containing through-hole openings is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block, though the xe2x80x9ctopxe2x80x9d designation may also be applied to the side opposite the surface mount contacts when referring to a filter in the board mounted orientation. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is affected, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions of the electromagnetic fields within and around the block are complex and difficult to predict.
These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and other components of the RF application device.
Although such RF signal filters have received widespread commercial acceptance since the 1980s, efforts at improvement on this basic design have continued.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of filter technology to provide sufficient frequency selectivity and band isolation.
Coupled with the higher frequencies and crowded channels are the consumer market trends towards ever smaller wireless communication devices (e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators or allow greater insertion loss in order to provide improved signal rejection.
Moreover, the consumer market applies constant downward pressure on the price of portable wireless communication devices, which in turn puts downward pressure on related RF components such as filters. Thus there continues to be a need for lower cost monoblock filter designs as well as methods for making lower cost filters.
A cost driver in ceramic monoblock filter manufacturing is application of the conductive outer coating. Silver-based coatings are preferred for superior conductivity, but remain a costly choice for providing filter metallization. Gold coatings are technically feasible, but cost prohibitive. Copper coatings are a poorer-performing, but a less costly, alternative to silver.
One approach to producing monoblock ceramic filters includes dipping a shaped ceramic block into a viscosity-controlled slurry of silver particles to coat the ceramic block and through-holes. The dipped block is then heated to bind the silver particles together and to adhere the silver to the surfaces of the ceramic block.
The slurry dipping processes provide a silver coating which is thinner at block edges and thicker towards surface centers. For adequate filter performance and manufacturing yield, the filter conductive coatings must be sufficiently thick on all metallized pattern regions. To ensure sufficient coverage at the block edges, the resulting filters are over-coated with silver away from the edges.
Accordingly, this invention pertains to providing monoblock silver-coated dielectric filters having a more uniform silver coating layer and therefore less costly silver waste.
This invention overcomes problems of the prior art by providing a communication signal filter comprising a rigid dielectric core having at least one pair of opposing sides. The core defines a plurality of through-hole passages extending between the opposing sides. Present on the core is a pattern of metallized and unmetallized regions. The metallized regions of the pattern include an inner layer of nickel and an outer silver-containing layer on the inner layer.
A method aspect of the present invention creates a monoblock communication filter having a dielectric core selectively plated with a relatively thin conductive silver on nickel coating. For example, a core of dielectric material is provided having at least one pair of opposing sides and defining a plurality of through-hole passages extending between the opposing sides. The communication filter is then created by depositing a nickel-based layer on the core, depositing a silver-containing layer over the nickel-based layer to form a plated core, patterning the plated core by removing a portion of the nickel and silver layer to form a pattern of unmetallized areas, and thereafter heating the patterned core at a temperature sufficient to melt a portion of the nickel-based layer.
The outer surface and through-hole sidewalls of the core are preferably etched with an acid solution before nickel plating. The nickel-based layer is preferably deposited on the core by contacting the core with an electroless nickel plating solution. The silver-containing layer is preferably provided by electrolytic deposition.
In a preferred embodiment of the resulting communication filter, the silver-containing layer and the nickel layer form a diffused interface therebetween substantially coextensive with the nickel layer.
An alternate embodiment of the present invention is a duplexing communication signal filter adapted for connection to an antenna, a transmitter and a receiver for filtering an incoming signal from the antenna to the receiver and for filtering an outgoing signal from the transmitter to the antenna. The duplexing filter comprises a rigid core of dielectric material with a top surface and a bottom surface and defining a series of through-holes. Each through-hole extends from an opening on the top surface to an opening on the bottom surface. Present on the core of the duplexing filter is a surface-layer pattern of metallized and unmetallized areas. The metallized areas each have a first layer of nickel and a second silver-containing layer on the first layer. The pattern includes a relatively expansive metallized area for providing off-band signal absorption, an unmetallized area circumscribing at least one of the openings, a transmitter connection metallized area, a receiver connection metallized area, and an antenna connection metallized area positioned between the transmitter connection metallized area and the receiver connection metallized area.
There are other advantages and features of this invention which will be more readily apparent from the following detailed description of preferred embodiments of the invention, the drawings, and the appended claims.